1. Understand the molecular mechanisms underlying

early embryonic development in vertebrates.

2. Explain, in general, how organizers function to

pattern the forming axes of the early embryo.

3. Appreciate the conservation of molecular

mechanisms controlling body plan development in

different organisms: the case of homeotic genes.

4. Colinearity of the homeotic genes in man.

Learning Outcomes

Developmental processes occurring during vertebrate development

Axes formation-Signalling centres

Left right asymmetry

Anterior-posterior axis formation

Outline

Animals must be specified in three dimensions

The germ layers are created during gastrulation

Lecture E01

The germ layers form different tissues

Basic morphogenic processes are similar between animals

Gastrulation in a fly

FlyBase

Development in vertebrates is based on cell-cell interactions:

groups of cells called organizing centres emit instructive signals that induce and pattern surrounding tissues.

The concentration gradient of the (signal) morphogen induces multiple cell choices. (E05)

Organisers are involved in body axis formation in vertebrates

Signalling centres instruct surrounding cells to form tissues

Node graft

Two headed cow...

Genetic determinants involved in body axis formation in mammals

The major signalling centre in vertebrates is the node

Node

Chicken Human

Question: How does the node pattern?

Genetic determinants involved in body axis formation in mammals

Organisers ‘pattern’ surrounding cells and tissues by secreting

signaling molecules (proteins)

Node cells secretes nodal and noggin and FGF

FGFNodal

Cells signalling through transmembrane receptors

SHCGrb2

SOSRAS

RAF

MEK

MAPK

FGF

FGFR Extracellular

Intracellular

P

Genetic determinants involved in body axis formation in mammals:Neural tissue

Signalling centres instruct surrounding cells to form tissues

Overlying tissues form a neural tube

Node or FGF protein

Gradients of secreted proteins produce the different germ layers

Left-right asymmetry of internal organs

LungsHeartGut loopingLiver

http://mekhala.blogspot.com/2007_11_25_archive.html

Left-right patterningasymmetric signalling from the nodeThe expression of genes on the left side of the embryo leads to a cascade of gene expression and morphogenic changes

Nodal

Pitx2

Nodal

chickGut looping, heart looping

In situ hybridisations of left-right asymmetry genes

Node and ciliaHow to break symmetry

anterior

posterior

R L

The node spins

Loss of left-right asymmetry leads to disease

Situs inversus

•Named for mutations thatrevealed existence

•Bithorax – part of haltere on 3rd thoracic segment is transformed into part of a wing

•Antennapedia – dominant mutations transform antennae into legs

•Homeotic mutation is the transformation of one segment into another related one

Homeotic genes

Colinearity: location on the chromosome corresponds to the spatial expression pattern

3’5’

Temporal and spatial colinearity: order of Hox genes on the chromosome follows the antero-posterior body axis.

Veraksa, Del Campo & McGinnis. 2000. Mol. Genet. Metab., 69, 85-100.

How do we get anterior-posterior axis: the HOX Genes!!

Combinations of Hox genes specify the development of the anterior-posterior axis

Hox gene expression follows the somite bondaries

Embryonic structures Adult organs

Film of somitogenesis

When Something Goes Wrong…

Lumbar vertebra

Thoracic vertebra

Extra rib

The function of Homeotic genes in mammals is similar to in flies: the KO of hoxc8 in mouse causes an homeotic transformation: the first lumbar vertebra forms a rib.

*

Summary: patterning of the vertebrate axial body plan

the four Hox gene complexes are expressed along the antero-posterior axis

Hox gene expression establishes positional identity for mesoderm, endoderm, and ectoderm

gastrulation and organizer activity

mesoderm develops into notochord,somites, and lateral plate mesoderm

somite develops into sclerotomeand dermomyotome

mesoderm induces neural plate from ectoderm

notochord patterns neural tube

Polydactyly

Diseases associated with Hox gene mutations

1. Hand-foot-genital syndrome (Hox A11-13 deletion)

2. Synpolydactyly (HoxD13 deletion)

3. Cleft palate4. Brain abnormalities5. Leukemia (Hox D4)6. Retinoic acid, which causes

birth defects, affects Hox genes

Teratology Lecture

Hox genes and vertebrate segment identity

•Hox gene mutations lead to subtle phenotypesWhy??

•Hox genes are used over and over again in the developing embryo

>>>Multiple phenotypes, multiple cancers

Reference book: Developmental Biology, Gilbert